The big picture: Community ecology


  • Community:
    • small scale: decomposers on a fallen log
    • large scale: flowers + pollinators in Shenandoah national park


  • Interactions can help, harm or have no effect


  • Outcome of interactions determine which species are present


  • Sum of interactions determine how an ecosystem functions
    • productivity, water quality, population control

Inter-specific Competition (-/-)



  • Individuals sharing space will compete for resources
    • reduces fitness for each species


  • Resources can mean many things:
    • prey items, water, nutrients, space


  • Competitive exclusion:
    • local extinction
    • rare in nature (sometimes on islands)

Competitive exclusion: squirrel species in the UK

The Eurasian red squirrel has suffered competitive exclusion due to the introduction of the Eastern grey squirrel from the USA

Natural selection favors those that do not compete


  • Ecological niche:
    • abiotic factors, food choice, nest sites
    • competition occurs for these resources


  • Natural selection reduces overlap between niches!
    • resource partitioning: division of limited resources


  • Resource division allows for coexistence
    • use parts of a habitat, active at different times
    • allows partial niche overlap (not full)
    • why competitive exclusion is rare!

Resource partitioning in warblers

There is serious DRAMA in the animal kingdom!!!




  • Review: all animals are heterotrophic….


  • All animals must eat, which means that most organisms are also at the risk of being eaten


  • Exploitation: species feeding interactions (+/-)
    • predation, herbivory & parasitism
    • the drama drives natural selection!

Predation (+/-)




  • One species (predator) kills and eats other (prey)


  • Lots of feeding adaptations in the animal world
    • fangs, claws, echo-location, venom, etc. etc. etc.


  • Fitness (reproductive success) depends on feeding

Predation (+/-)



  • Not getting eaten is just as important as eating
    • powerful driver of natural selection


  • Behavioral adaptations
    • hiding, fleeing, herding
    • active self defense less common


  • Morphological adaptations
    • spines, odors, toxin accumulation
    • mimicry and camouflage

Defenses vs Predation → Co-evolutionary arms race

https://youtube/w-K_YtWqMro

Herbivory (+/-)


  • Organism (herbivore) eats part of plant or alga
    • harms the plant by removing biomass


  • Herbivores are large & small, terrestrial & aquatic


  • Like predators, many special adaptations
    • chemical sensors to identify toxins
    • specialized teeth or digestive systems


  • Review: Plants don’t move
    • just as many defensive adaptations
    • thorns, toxins, etc.

Parasitism (+/-)




  • Organism (parasite) gets nourishment from host
    • host harmed but rarely killed
    • ~1/3 of all species are parasites!!


  • Parasites live inside or outside body of host
    • tapeworms to ticks


  • Parasites usually have complex life cycles
    • feeding and reproductive cycles using host

Species interactions are not all bad!





  • Positive interactions occur where one at least one species benefits and their other is not harmed


  • Positive interactions common in ecological communities
    • drive higher biodiversity


  • Mutualism and commensalism are the most common

Mutualism (+/+)




  • Occur when both species benefit from the interaction
    • common in nature


  • The 2 species may or may not depend on each other for survival/reproduction
    • plants + insects = pollination + food


  • Usually still a cost, but benefit > costs
    • carbon - nitrogen trade-off between plants and fungi

Commensalism (+/0)




  • Commensalisms are interactions where one benefits but doesn’t harm/help the other
    • also common in nature


  • Some biologists argue that any close interaction between two organisms is unlikely to be completely neutral for either party
    • especially over time

Trophic (feeding) structure defined by species interactions



  • Structure and dynamics of a community depend on feeding relationships


  • Energy is transferred upward
    • plants/aglae → consumers
    • transfer is wasteful (most energy is lost)


  • Trophic level:
    • many species can share a trophic level

Trophic (feeding) structure defined by species interactions



  • Structure and dynamics of a community depend on feeding relationships


  • Energy is transferred upward
    • plants → consumers
    • transfer is wasteful (WHY?)


  • Exists in terrestrial and aquatic systems
    • What happens if a species goes extinct?

Food webs are multi-dimensional

Some species in a community are more impactful than others




  • Dominant species:
    • better @ obtaining resources
    • defense wins championships


  • Keystone Species:


  • Ecosystem engineers:
    • impact every community member